The wastewater of one third of Ireland’s population is treated on-site using domestic treatment systems (DWWTSs) that usually consist of a septic tank and soil attenuation system. Within the past four years, the legislative framework for these systems has undergone a major change with a registration and inspection regime being introduced to identify legacy sites that will require remediation work, particularly in areas of the country underlain by subsoils of very low permeability. Against this background this study aims to assess the overall sustainability of existing DWWTSs as well as alternative treatment and disposal options. The results show that main CO2eq
emissions are from the methane production in septic tanks. The reduced methane production in mechanically aerated secondary treatment systems was found to counterbalance the related emissions due to the additional energy requirements. In contrast, septic tank systems have the lowest construction and operational costs representing the most economically sustainable solution. Pressurised disposal systems are slightly more expensive but have the potential to reduce environmental impact on surface water and reduce greenhouse gas emissions. Clustered decentralised treatment solutions could be environmentally and economically sustainable but ownership, management and related financial and legal issues will need to be addressed and developed.